In list context, this sorts the LIST and returns the sorted list value.
In scalar context, the behaviour of sort() is undefined.
If SUBNAME or BLOCK is omitted, sorts in standard string comparison
order. If SUBNAME is specified, it gives the name of a subroutine
that returns an integer less than, equal to, or greater than 0
,
depending on how the elements of the list are to be ordered. (The <=>
and cmp
operators are extremely useful in such routines.)
SUBNAME may be a scalar variable name (unsubscripted), in which case
the value provides the name of (or a reference to) the actual
subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
an anonymous, in-line sort subroutine.
If the subroutine's prototype is ($$)
, the elements to be compared
are passed by reference in @_
, as for a normal subroutine. This is
slower than unprototyped subroutines, where the elements to be
compared are passed into the subroutine
as the package global variables $a and $b (see example below). Note that
in the latter case, it is usually counter-productive to declare $a and
$b as lexicals.
The values to be compared are always passed by reference and should not be modified.
You also cannot exit out of the sort block or subroutine using any of the
loop control operators described in perlsyn or with goto.
When use locale
is in effect, sort LIST
sorts LIST according to the
current collation locale. See perllocale.
sort() returns aliases into the original list, much as a for loop's index
variable aliases the list elements. That is, modifying an element of a
list returned by sort() (for example, in a foreach
, map or grep)
actually modifies the element in the original list. This is usually
something to be avoided when writing clear code.
Perl 5.6 and earlier used a quicksort algorithm to implement sort. That algorithm was not stable, and could go quadratic. (A stable sort preserves the input order of elements that compare equal. Although quicksort's run time is O(NlogN) when averaged over all arrays of length N, the time can be O(N**2), quadratic behavior, for some inputs.) In 5.7, the quicksort implementation was replaced with a stable mergesort algorithm whose worst-case behavior is O(NlogN). But benchmarks indicated that for some inputs, on some platforms, the original quicksort was faster. 5.8 has a sort pragma for limited control of the sort. Its rather blunt control of the underlying algorithm may not persist into future Perls, but the ability to characterize the input or output in implementation independent ways quite probably will. See sort.
Examples:
- # sort lexically
- @articles = sort @files;
- # same thing, but with explicit sort routine
- @articles = sort {$a cmp $b} @files;
- # now case-insensitively
- @articles = sort {uc($a) cmp uc($b)} @files;
- # same thing in reversed order
- @articles = sort {$b cmp $a} @files;
- # sort numerically ascending
- @articles = sort {$a <=> $b} @files;
- # sort numerically descending
- @articles = sort {$b <=> $a} @files;
- # this sorts the %age hash by value instead of key
- # using an in-line function
- @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
- # sort using explicit subroutine name
- sub byage {
- $age{$a} <=> $age{$b}; # presuming numeric
- }
- @sortedclass = sort byage @class;
- sub backwards { $b cmp $a }
- @harry = qw(dog cat x Cain Abel);
- @george = qw(gone chased yz Punished Axed);
- print sort @harry;
- # prints AbelCaincatdogx
- print sort backwards @harry;
- # prints xdogcatCainAbel
- print sort @george, 'to', @harry;
- # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
- # inefficiently sort by descending numeric compare using
- # the first integer after the first = sign, or the
- # whole record case-insensitively otherwise
- @new = sort {
- ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
- ||
- uc($a) cmp uc($b)
- } @old;
- # same thing, but much more efficiently;
- # we'll build auxiliary indices instead
- # for speed
- @nums = @caps = ();
- for (@old) {
- push @nums, /=(\d+)/;
- push @caps, uc($_);
- }
- @new = @old[ sort {
- $nums[$b] <=> $nums[$a]
- ||
- $caps[$a] cmp $caps[$b]
- } 0..$#old
- ];
- # same thing, but without any temps
- @new = map { $_->[0] }
- sort { $b->[1] <=> $a->[1]
- ||
- $a->[2] cmp $b->[2]
- } map { [$_, /=(\d+)/, uc($_)] } @old;
- # using a prototype allows you to use any comparison subroutine
- # as a sort subroutine (including other package's subroutines)
- package other;
- sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
- package main;
- @new = sort other::backwards @old;
- # guarantee stability, regardless of algorithm
- use sort 'stable';
- @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
- # force use of mergesort (not portable outside Perl 5.8)
- use sort '_mergesort'; # note discouraging _
- @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
If you're using strict, you must not declare $a
and $b as lexicals. They are package globals. That means
if you're in the main
package and type
- @articles = sort {$b <=> $a} @files;
then $a
and $b
are $main::a
and $main::b
(or $::a
and $::b
),
but if you're in the FooPack
package, it's the same as typing
- @articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it returns
inconsistent results (sometimes saying $x[1]
is less than $x[2]
and
sometimes saying the opposite, for example) the results are not
well-defined.
Because <=>
returns undef when either operand is NaN
(not-a-number), and because sort will trigger a fatal error unless the
result of a comparison is defined, when sorting with a comparison function
like $a <=> $b
, be careful about lists that might contain a NaN
.
The following example takes advantage of the fact that NaN != NaN
to
eliminate any NaN
s from the input.